Unlock RuboCop version

Ransack supports Ruby 2.6 or higher then RuboCop version can be unlocked.

.rubocop.yml

@@ -1,6 +1,5 @@
 AllCops:
-  # TargetRubyVersion value is based on `required_ruby_version = '>= 2.3'` in the ransack.gemspec
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.6
 
   DisabledByDefault: true
 

Gemfile

@@ -50,7 +50,4 @@ group :test do
   gem 'simplecov', :require => false
 end
 
-# RuboCop 0.81.0 is the last version which supports Ruby 2.3.
-# Once Ransack required_ruby_version is bumped, RuboCop version can be bumped.
-# https://github.com/rubocop-hq/rubocop/pull/7869
-gem 'rubocop', '=0.81.0', require: false
+gem 'rubocop', require: false